THE CUTTING EDGE | SCIENCE WATCH

'Electronic Tongue' Offers a Taste of the Future

Scientists and engineers at the University of Texas in Austin are developing what they call an "electronic tongue" that can taste the difference between hundreds of substances. Aided by a recent $600,000 grant from the National Institutes of Health, the researchers believe they are paving the way for cheap, disposable sensors that can tell a doctor nearly everything that is in a patient's blood or urine.

The device uses tiny polymer spheres--taste buds, if you will--embedded in a silicon wafer that can recognize and quantify everything from blood sugar to drugs. The sensors distinguish between subtle flavors by using a combination of the four elements of taste--sweet, sour, salty and bitter.

"It's really quite feasible to have an array of sensors that will do 25 or 50 tests all at once," says Dean Neikirk, professor of electrical and computer engineering at the university. He sees the "electronic tongue" as an eventual replacement for individual tests that target specific substances, such as cholesterol or sugar.

Although the immediate application would most likely be medical, the device has all sorts of potential uses, Neikirk says, ranging from analyzing contaminated water to determining which substances make drinks taste the best.

The research came about serendipitously, he says. Neikirk and two colleagues in the chemistry department, John McDevitt and Eric Anslyn, were jawboning a couple of years ago about their very different fields.

McDevitt and Anslyn had developed tiny spheres, or beads, in which they embedded sensors that change color in the presence of certain liquids. Similar to the litmus test, one type of sphere turned from purple to yellow in the presence of high acidity, and others responded to other conditions with different colors. By reading the colors through a camera-on-a-chip connected to a computer, the chemists were able to determine exactly what was present, and in what quantities.

But there was a problem.

The spheres act like sponges, absorbing the liquid and nearly doubling in size. How could you fabricate a device that would hold the expanding sensors in place without destroying them?

The answer, Neikirk believed, was right in his lab.

"I use micro-machining to make little mechanical structures that are about a tenth of a millimeter in size, and that's about how big their beads are," he says. The structures are like tiny wells in a microchip, and he creates them by the silicon etching process used to make integrated circuits.

The marriage between these two very different fields has led to a prototype, and work is now underway to produce arrays of sensors that can conduct many tests simultaneously. The ultimate goal, Neikirk says, is to produce the arrays on something similar to Scotch tape, which can be used and discarded.

Neikirk thinks that is feasible, and at a very low cost.

Chemical sensors, of course, are not new. All types of sensors are now available, but most are programmed to test for the presence of a single substance, and the results are not always conclusive. The National Institutes of Health grant is based on the idea that the array could do many reliable tests at once on blood and urine samples.

The electronic tongue is only one of several engineering devices that could ultimately lead to robots that can perform many human-like chores. That may not be the stated goal of any of the research, but it is hard to argue that they are not leading us in that direction.

At the California Institute of Technology in Pasadena, for example, chemist Nathan Lewis is developing a mechanical nose that can sniff out about 100 odors instantly. The Caltech nose, called Pinocchio, consists of a series of electrical resistors that are chemically sensitive to vapors.

Lewis believes the device will ultimately be able to sniff out almost anything, eventually replacing the drug-sniffing dogs at the airport and even telling which wine is good and which is bad.

Scientists at Harvard University and the Massachusetts Institute of Technology are developing microchips that could someday be placed in a damaged human eye, curing some types of blindness. The hope is to replace a malfunctioning sensor on the retina with an artificial sensor--a silicon chip.

I can see it now. A robot scans a slimy lagoon, keeping an eye on a deformed frog. It slips into the murky depths, sniffing the frog's odor as it quietly moves underwater. It grabs the frog, squeezes out a little urine, and instantly tests for the presence of dangerous contaminants.